This invention relates to variable displacement axial piston fluid devices, such as pumps or motors, and particularly to a system for lubricating the bearing support surfaces of an adjustable position swashblock used in such devices.
One type of variable displacement axial piston pump or motor uses a shaft mounted rotating barrel having a plurality of parallel cylinders each containing a piston. The pistons each mount a shoe at one end that rides against a flat surface of a swashblock. The swashblock is movable so that the surface can be positioned at an angle to a plane normal to the axis of rotation of the barrel. As the barrel is rotated, the pistons reciprocate within the cylinders and the shoes slide over the angled swashblock surface. The angle of the surface will determine the volume displaced by each piston. When the barrel of a pump is rotated, fluid is drawn in through a low pressure port and is pumped out of a high pressure port. When fluid under pressure is pumped into the high pressure port, the barrel will be rotated so that the device will function as a fluid motor.
The swashblock is either mounted on trunnions or it has its rear face formed as a portion of a circular cylinder that mates with a similarly curved support. In the later case, the mating surfaces of the swashblock and its support are subjected to large forces transmitted through the pistons and shoes as the cylinders are exposed to the high pressure port. Often the mating surfaces of the swashblock and its support are metal-to-metal and this large force causes a great amount of friction that must be overcome to pivot the swashblock to adjust the displacement of the pump. The conventional solution has been to supply fluid under high pressure to the interface between the swashblock and its support either by use of an exterior high pressure line that leads from the high pressure port to the interface (U.S. Pat. No. 3,682,044) or by pumping high pressure fluid through passages in the pistons, the shoes and the swashblock to the interface (U.S. Pat. No. 3,898,917). In either case, the result is that high pressure fluid is pumped to the interface between the swashblock and the support to create a counterbalancing force.
Another approach interposes a bearing material between the mating surface and the support and the fluid within the pump housing is relied upon to lubricate the bearing. However, under high forces the bearing aligned with the high pressure port may be subjected to such a high axial force that the lubricating fluid will migrate away from the area of greatest stress and the bearing becomes dry. If this occurs, the force required to pivot the swashblock can rise to an unacceptable level.
The present invention provides a system to insure the delivery of lubricating fluid to a bearing over the entire bearing surface of the swashblock.